Liquefied natural gas (hereinafter referred to as LNG) has become an industrially important material. It is commonly used as a supplemental source of natural gas because of the relative ease of storing gas as liquid instead of in the gaseous form. In this way gas may be stored and later used for so-called "peak shaving" during periods when gas demand is very high. Another major reason for the importance of LNG is that natural gas is often found in remote locations, where it is of little value. Consequently, the natural gas may often be wasted. In order to utilize this gas, it is necessary to transport it to known markets. Although the transportation of gas by pipeline normally takes place over fairly lengthy distances, this would be no problem where only transportation over land is encountered. However, in many instances the natural gas is separated from its natural market by expansive bodies of water. As a result large plants have been developed at ports of embarkation to liquefy the natural gas for transportation by ship to the foreign markets. There the LNG may be vaporized and the gas directly introduced into the gas distribution system as it is required. However, if the LNG is to be moved to points somewhat remote from the shore terminals at which it is received, a decision must be made as to whether the LNG will be transported in the gaseous phase, as a liquid or by some other means.
The present invention will apply typically to the transportaton of LNG between the shoreline terminals where it is received from overseas locations by tanker and the point of ultimate use, which may be a gas distribution system many miles from the shore terminal.
The prior art has indicated several ways of accomplishing the aforementioned result. One way of transporting the natural gas would be to vaporize it at the shore terminal and transport it entirely by gas pipeline, as would be done if the gas had never been in the liquefied form. Another possibility would be transportation by rail or by truck. Still yet another possibility would be transportation by an LNG pipeline, that is to say, keeping liquid at all times throughout the pipeline so that it is received as a liquid at the distribution point. From there it would be vaporized for use by the ultimate consumer. Such a pipeline, discussed in U.S. Pat. No. 3,256,705, would necessarily be heavily insulated since LNG has an atmospheric boiling point of about -259.degree. F. The pipeline would require intermediate cooling and repumping stations in order to maintain the gas in the liquid state. A continual heat in-leak would be expected throughout the pipeline which would vaporize the LNG and thereby create practical problems in its handling as well as reducing the capacity of the pipeline unless additional cooling were used.
An alternative method, which has been suggested in U.S. Pat. No. 2,958,205, would be to pump LNG, but instead of cooling it to prevent vaporization caused by heat in-leak deliberately allowing a certain portion of the LNG to vaporize to compensate for the heat in-leak. At predetermined locations the mixture of gas and liquid would be removed from the pipeline and separated. The gaseous portion would be compressed and reliquefied and the liquid portion repumped. Following this, the streams would be recombined and the pipeline would operate with only a liquid phase again until a certain distance was covered and the heat in-leak would cause vaporization to begin again.
The present invention provides a novel means of utilizing the inherent characteristics of LNG and at the same time providing a novel and improved method for transportation of the LNG to the ultimate consumer as will be seen in the description which follows below.